This invention is related to an apparatus for minimizing the effects of differential sticking on drill string components and more particularly to an apparatus for minimizing the effects of differential sticking which utilizes the principles of operation of a conventional journal bearing.
In a conventional drilling operation wherein a drill string having a drill pipe and a drill collar with a drill bit on the lower end of the drill collar, is used, the drill string is continuously rotated and drilling fluids, such as drilling mud, are circulated down through the center of the drill string and out through the drill bit to be circulated back to the surface through the annular space between the outside of the drill string and the wellbore boundary to carry the cuttings from the drill bit out of the wellbore. During this drilling operation, the drill string is usually held in tension to control the weight on the bit and the pressure within the wellbore is maintained at a high level due to the flow of drilling mud, the static head or density of the drilling mud.
One problem which is often encountered in such an operation is that a differential pressure in the wellbore is created due to the high pressure of the drilling fluids as compared to the low pressures in the formations in which the wellbore is being drilled. This differential pressure problem is generally reduced by the drilling mud which acts to seal the porous part of the formation around the wellbore. However, in some cases, especially in the more porous formations, the drilling mud does not form a sufficient insulator between the drill pipe and the boundary of the wellbore. Therefore, it is especially important in this situation to maintain continuous rotation of the drill string within the wellbore so that a drilling mud boundary can be provided between the drill pipe and the side of the wellbore. Yet, instances do occur when the rotation of the drill ceases and pipe becomes stuck against the side of the wellbore due to the differential pressures involved. Often times, rotation of the drill pipe or retraction of the drill pipe from the wellbore becomes impossible, resulting in an expensive loss since the entire well is lost.
Many different drill pipe designs have been proposed to eliminate this differential sticking problem. Some drill collars have been made with a square cross section so that the surface area for contacting the side of the wellbore is minimized. Other designs have used spiral flutes formed in the side of the drill string sections to reduce the surface area of the drill pipe subject to differential sticking and to provide more access for the drilling mud to reach the formation behind the drill string section. While these design features may reduce the possibility of differential sticking, the possibility still exists.